JP2004156621A - Honeycomb filter for exhaust gas purification - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a honeycomb filter for exhaust gas purification which is characterized by large cracking margin, excellent strength against thermal stress and cracking phenomenon or the like, and small pressure loss generated inside the filter. <P>SOLUTION: The honeycomb filter is applied to an exhaust gas purifying honeycomb filter collecting particulate remaining in exhaust gas, and is applied to an alternative eye-sealing honeycomb filter formed by porous silicon carbide sintering member as the part of an exhaust emission control device arranged at an exhaust passage of an internal combustion engine employing fuel addition agent in order to decrease the particulate in the exhaust gas. <P>COPYRIGHT: (C)2004,JPO

Description

The present invention relates to a honeycomb filter for purifying exhaust gas, which is disposed between exhaust passages of an internal combustion engine using a fuel additive to reduce particulates in exhaust gas, and captures particulates remaining in the exhaust gas. Things.

Internal combustion engines such as diesel engines contain particulates (for example, soot and unburned fuel) in exhaust gas. In particular, diesel engines that use light oil as fuel and direct injection gasoline engines that have become popular in recent years have large particulate emissions. For this reason, it is known to remove the particulates by an exhaust gas purifying apparatus including an exhaust gas purifying alternate plugged honeycomb filter disposed between the exhaust passages of the internal combustion engine.

As the exhaust gas purifying honeycomb filter, a cordierite filter 32 in which a part of a honeycomb-shaped end face is plugged alternately is generally used as shown in a schematic sectional view of FIG. The cordierite filter 32 has a plurality of exhaust gas flow holes 33 extending in parallel with the longitudinal direction thereof, and one end of each of the flow holes 33 on the gas inflow side and the discharge side is alternately sealed with small pieces 33a. Have been.

When the exhaust gas Gin from the diesel engine flows into the cordierite filter 32 from the exhaust passage 11, the particulates in the exhaust gas are filtered at the surface of the exhaust gas flow hole 33. Then, the exhaust gas Gout that has been purified by passing through the cordierite filter 32 is discharged outside the vehicle again through the exhaust passage 11.

By the way, it is also known that a pressure loss ΔP occurs when the exhaust gas Gin passes through the cordierite filter 32. The pressure loss ΔP is represented by the following equation (1).
ΔP = ΔP1 + ΔP2 + ΔP3 + ΔP4 (1)
ΔP1: resistance caused by narrowing of an opening for flowing from the exhaust passage 11 into the exhaust gas flow hole 33 ΔP2: resistance generated when flowing through the exhaust gas flow hole 33 ΔP3: passing through the wall of the exhaust gas flow hole 33 .DELTA.P4 generated at the time of exhaust gas flow: Resistance generated when passing through the particulates accumulated on the surface of the exhaust gas flow hole 33

In this case, the resistances .DELTA.P1, .DELTA.P2 and .DELTA.P3 each depend on the cell structure of the cordierite filter 32, and are constant values .DELTA.Pi (hereinafter ".DELTA.P1 + .DELTA.P2 + .DELTA.P3" which are not dependent on the passage of time such as particulate deposition). Initial pressure loss "). Therefore, most of the pressure loss .DELTA.P is determined by the resistance .DELTA.P4 generated when passing through the particulate during deposition, and this resistance .DELTA.P4 is usually two to three times the initial pressure loss .DELTA.Pi when the particulate is deposited. Value.

FIG. 2 shows the relationship between the cell structure of the filter and the typical specifications, geometric surface area and aperture ratio of the filter. The cell structure Cs (mil / cpi) is represented by the cell wall thickness dc (mil = mili inch) with respect to the number of cells per square inch Nc (cpi = cells per squere inches), and the geometric surface area fs ( cm ² / cm ³ ) is represented by an area (filtering area) through which exhaust gas can pass per unit volume. In FIG. 2, the thickness dc of the cell wall is expressed in (mm) units.

According to FIG. 2, the pressure loss ΔP generated between the alternating plugged honeycomb filters for purifying exhaust gas decreases as the number of cells (cell) and the geometric surface area fs (cm ² / cm ³ ) of the filter increase. The opening ratio α (%) is the ratio of the opening area of all the gas flow holes to the filter cross-sectional area. According to FIG. 2, the limit for preventing cracks (the crack limit) is the opening ratio. It becomes larger as α is lower.

By the way, the mechanical strength of the filter, that is, the bending strength S * of the filter, is substantially equal to the strength when the filter is filled with the porous material multiplied by the relative density ρ * described later. If the properties of the porous material constituting the filter are density ρ and strength S, the bending strength S * and the relative density ρ * of the filter are
ρ * = α × ρ (2)
S * ≒ ρ * × S (3)
It becomes. That is, the smaller the aperture ratio α, the higher the strength.

In addition, since regeneration of the filter is performed by burning the particulates, strength against thermal stress is important. When the filter is made of ceramic, it causes brittle fracture due to thermal stress and causes cracks. Cracks are more likely to occur as the amount of heat generated during reproduction, that is, the amount of particulates, increases. This resistance is called a crack phenomenon and is defined by the amount of particulates. The crack limit is proportional to the aperture ratio α and is also closely related to the cell wall thickness dc. If the opening ratio α is the same, the crack limit is higher as the cell wall thickness dc is larger.

FIG. 3 shows the amount Qp (g / L) of particulates captured per unit volume on the vertical axis, and the flow velocity Va (m / s) of air passing through the filter cross section on the horizontal axis. . Here, a solid line A indicates a porous silicon carbide sintered body alternating plugged honeycomb filter, and a dashed line B indicates a cordierite alternating plugged honeycomb filter. These filters have the same cell structure Cs = 17/100 (mil). / Cpi).

When it is intended to burn the particulate matter of a predetermined amount Qp trapped on the filter surface by supplying air having a flow rate Va into the filter, as shown in FIG. 3, when the flow rate Va is very small, However, since the heat conduction to the particulates is not sufficient, the amount of the particulates burned is small and the crack limit is high. When the flow velocity Va rises to the predetermined value Vo, heat conduction is sufficiently performed, so that the burning amount of the particulates increases and the crack limit sharply decreases. Thereafter, when the flow velocity Va further rises, heat is taken backward by the air flow, so that the amount of particulate combustion decreases and the crack limit gradually increases.

Specifically, from FIG. 3, a porous cordierite alternating plugged honeycomb filter (dot-dash line B) and a porous silicon carbide sintered body (SiC) alternating plugged honeycomb filter (solid line A) having the same cell structure are shown in FIG. In comparison, the SiC filter can burn three times more particulates than the cordierite filter. That is, the crack limit of the SiC filter is three times higher than that of the cordierite filter.

Therefore, a filter having good characteristics is formed of a material having a large crack limit, excellent strength against thermal stress and crack phenomena, and a small pressure loss between filters.

In recent years, in order to suppress the generation amount of particulates in exhaust gas, for example, a fuel containing an additive in advance and a device for dropping the additive onto the fuel have been developed, and the use thereof has been increasing. These fuel additives have the effect of preventing the formation of soot and the like during the combustion of the fuel.

However, even if such an additive is used, the generation of particulates cannot be completely suppressed, and as a result, particulates are generated in the exhaust gas. Therefore, the use of an exhaust gas purification filter is also indispensable. .

で表される。なお、パティキュレートを構成する燃料の未燃分についても、有機化合物であるため、排気ガス浄化用フィルタの加熱により燃焼され再生される。 By the way, the exhaust gas purifying filter can be regenerated by burning the particulates. In particular, when soot burns,
C + O ₂ → CO ₂ + Q (calorific value) (4)

Is represented by The unburned portion of the fuel that constitutes the particulates is also an organic compound, so that it is burned and regenerated by heating the exhaust gas purification filter.

Here, since the fuel additive has the effect of reducing the activation energy Ea, the particulate contained in the exhaust gas starts burning at a low temperature, as is apparent from the above equation (5). For this reason, most of the particulates contained in the exhaust gas can be efficiently reduced by combustion in the internal combustion engine and heating of the exhaust gas purifying filter.

FIG. 4 shows the pressure loss ΔP (mmAq) generated when cordierite is used as an alternating plugged honeycomb filter for purifying exhaust gas, and the temperature T (° C.) in the filter in time t (min). Experimental data. In FIG. 4, reference symbol Po denotes combustion with a fuel additive. When the temperature To increases with an increase in the engine speed (engine load), the accumulation of particulates starts to decrease after a certain time. . That is, particulate combustion starts at a certain temperature To = 380 (° C.), and the filter is regenerated.

On the other hand, the symbol Pn indicates combustion without a fuel additive, and even if the temperature Tn increases with an increase in the engine speed (engine load), the pressure loss between the honeycomb filters is proportional to the accumulation of particulates. Pn also rises continuously. For this reason, unlike the combustion with the fuel additive, the combustion of particulates is not performed at a temperature around Tn = 380 (° C.). In the case of combustion without a fuel additive, the combustion start temperature of particulates is generally around Tn = 630 (° C.).

By the way, in the prior art, cordierite is employed as the alternating plugged honeycomb filter for purifying exhaust gas. However, since the cordierite alternating plugged honeycomb filter has a low maximum operating temperature, there is a problem that the amount of particulates that can be processed in one regeneration is limited. In this case, since a large pressure loss occurs between the filters due to the accumulated particulates, the combustion efficiency of the internal combustion engine is reduced, and the fuel efficiency is deteriorated. Further, when the particulates are burned, a large change in pressure loss occurs, which causes the driver to feel uncomfortable.

For this reason, referring to FIG. 2 as a means for reducing the pressure loss generated between the alternating plugged honeycomb filters for exhaust gas purification, the cell structure Cs of the filter should be set finely, that is, the number of cells per unit square inch should be reduced. It is conceivable to set the value of the cell wall thickness dc (mil) small while setting the value of Nc cpi) large.

However, in the conventional cordierite alternately plugged honeycomb filter, there is a limit in making the cell structure finer due to a problem regarding the original strength of cordierite. For example, in a cordierite honeycomb filter, a filter having a cell number Nc of more than 100 (cpi) per square inch can be manufactured, but an alternate plugged honeycomb filter capable of efficiently burning particulates is used. In this case, the number of cells cannot be made larger than 100 (cpi) due to the problem of the crack limit.

The present invention has been made in view of the above-described facts, and has been achieved by employing a porous silicon carbide sintered body that can be set to a fine cell structure in an internal combustion engine using an additive-containing diesel fuel. An object of the present invention is to provide a honeycomb filter for purifying exhaust gas, which has a large crack limit, has excellent strength against thermal stress and crack phenomena, and has a small pressure loss between filters.

In order to solve this problem, the exhaust gas purifying honeycomb filter of the present invention is disposed between the exhaust passages of an internal combustion engine using a fuel additive to reduce particulates in the exhaust gas. An exhaust gas purifying honeycomb filter for capturing remaining particulates, which is an alternating plugged honeycomb filter formed of a porous silicon carbide sintered body.

In this case, since the exhaust gas purifying honeycomb filter is an alternating plugged honeycomb filter formed of a porous silicon carbide sintered body, the crack limit is higher than that of cordierite, and the strength against thermal stress and crack phenomenon is reduced. Excellent. In addition, the alternating plugged honeycomb filter can have a cell structure in which the number of cells per unit square inch is more than 100 and the cell wall thickness is less than 17 (mil), so that the pressure loss is lower than that of cordierite. Small enough.

Therefore, according to the present invention, the filter has a high crack limit and has high durability against thermal stress and crack phenomena. The pressure loss that occurs between them can be reduced. In addition, since the pressure loss generated between the exhaust gas purifying honeycomb filters is reduced, fuel efficiency can be improved.

In the exhaust gas purifying honeycomb filter, it is preferable that the total volume of the honeycomb filter is 1/4 to 2 times the total displacement of the internal combustion engine.

The total volume of the exhaust gas purifying honeycomb filter needs to be set to an appropriate value with respect to the total displacement of the internal combustion engine. However, if the total volume of the filter is too small, the pressure loss increases, and the fuel efficiency is significantly deteriorated. Conversely, if the total volume of the filter is too large, it may not be possible to install the filter in the exhaust passage due to layout problems.

Therefore, considering that the total volume of the filter is determined by the total displacement of the internal combustion engine, it is preferable to set the total volume of the filter to 1/4 to 2 times the total displacement of the internal combustion engine. In this case, the exhaust gas purifying device can suppress the pressure loss generated between the filters and can solve the layout problem that occurs when the exhaust gas purifying device is disposed between the exhaust passages of the internal combustion engine.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 5 shows an exhaust gas purifying apparatus used for a diesel vehicle.
An exhaust gas purifying apparatus 20, which is an embodiment of the exhaust gas purifying apparatus using the exhaust gas purifying honeycomb filter of the present invention, includes an exhaust passage 11 of a diesel engine using a diesel fuel containing an additive (for example, Eolys). And an exhaust gas purifying honeycomb filter 22 made of a porous silicon carbide sintered body (SiC) for capturing particulates contained in the exhaust gas Gin.

Here, the exhaust gas purifying honeycomb filter 22 of the present invention will be described in detail. The exhaust gas purifying honeycomb filter 22 is formed in a honeycomb shape by a porous silicon carbide sintered body. The exhaust gas purifying honeycomb filter 22 is formed with a plurality of exhaust gas flow holes 23 extending in parallel with the longitudinal direction, and one end of each of the flow holes 23 on the gas inflow side or the discharge side is formed of porous silicon carbide. The plugs are alternately plugged with small pieces 23a of the sintered body. For this reason, the exhaust gas purifying honeycomb filter 22 is formed of a porous silicon carbide sintered body, and the gas inlet side end face and the gas outlet side end face show a checkered pattern of cell wall thickness dc in a SiC alternating plugged honeycomb. It is a filter (see FIG. 6).

When the fuel containing the additive is used, the harmful components contained in the exhaust gas are reduced, but are not completely removed. Therefore, it is preferable that an exhaust gas purifying catalyst is carried on the inner wall surface of the exhaust gas flow hole 23 to remove these harmful components. As the exhaust gas purifying catalyst, various conventionally known catalysts can be used. The exhaust gas purifying honeycomb filter (hereinafter referred to as “SiC alternating plugged honeycomb filter”) 22 is a heat insulating material 23b provided on the outer peripheral surface thereof. And is tightly held in the casing 24 via.

The SiC alternating plugged honeycomb filter 22 according to the present invention has a cell structure in which the number of cells Nc per unit square inch is 120 or more and the cell wall thickness dc is 0.41 (mm) or less. Desirably, it is formed of a sintered body. Specific examples of the numerical value of the cell structure Cs (mil / cpi) include Cs = 14/200, 12/200, and 12/300 (mil / cpi).

Next, the operation of the present embodiment will be described.

When the exhaust gas Gin discharged from the diesel engine flows into the SiC alternating plugged honeycomb filter 22 in a state where the particulates contained in the exhaust gas are reduced by the additive added to the diesel fuel, the remaining particulates are exhausted. It is filtered on the surface of the gas flow hole 23. As a result, the exhaust gas Gout that has been purified by passing through the SiC alternating plugged honeycomb filter 22 is discharged to the outside of the vehicle again through the exhaust passage 11.

However, when the diesel engine is operated for a long time, particulates accumulate on the inner wall surface of each flow hole 23.

In addition, when a predetermined amount of the particulates is captured, it becomes difficult for the exhaust gas to pass through the flow holes, and the temperature inside the purification device rapidly rises. When the temperature reaches a predetermined temperature with this temperature rise, the particulates react with oxygen contained in the exhaust gas Gin and are incinerated.

FIG. 7 is a data diagram comparing the pressure loss generated in the SiC alternating plugged honeycomb filter used in the exhaust gas purifying apparatus of the present invention with the pressure loss generated in the conventional cordierite alternating plugged honeycomb filter. However, the cordierite alternating plugged honeycomb filter has a cell structure of 17/100 (mil / cpi), and the SiC alternating plugged honeycomb filter has 17/100 (mil / cpi), 14 It has a cell structure of / 200 (mil / cpi) or 12/300 (mil / cpi).

First, when the particulates remaining in the exhaust gas Gin are captured by the conventional cordierite filter, the pressure loss Pc reaches an equilibrium state when the pressure loss Pc exceeds 1750 (mmAq). Thereafter, since the particulates captured by the filter start burning, the pressure loss Pc is greatly reduced. If the fluctuation of the pressure loss is large, the feeling during operation is poor. In particular, when the driver depresses the accelerator pedal and the exhaust gas temperature T rises, the pressure loss suddenly decreases along with the rapid combustion of the particulates, so that the engine rotation increases in an unexpected situation of the driver.

In the SiC alternating plugged honeycomb filter of the present embodiment, since the diesel fuel contains an additive, the pressure loss P1 is in an equilibrium state at a temperature T = 380 (° C.), that is, when the pressure loss P1 is This is a time around 1250 (mmAq). Therefore, after this, the particulate matter trapped in the filter starts combustion at an early stage, so that the pressure loss P1 only slightly decreases, and the rate of decrease of the pressure loss P1 is also reduced by the cordierite alternate plugging having the same cell structure. Smaller than a honeycomb filter.

FIG. 8 is a data diagram showing the abundance rates of pores having different diameters in cordierite and SiC. As is clear from this figure, the SiC shown by the solid line shows a high abundance near a certain pore diameter, whereas the cordierite shown by the dashed-dotted line has a small pore diameter side and a large pore diameter side. 2 shows a high abundance rate. That is, while SiC has a structure having a substantially uniform pore diameter, cordierite has a structure having a non-uniform pore size. FIG. 9 shows this as an internal structure.

FIG. 9A is a schematic sectional view showing the internal structure of SiC, and FIG. 9B is a schematic sectional view showing the internal structure of cordierite. Since the SiC shown in FIG. 9A is constituted by the continuous ventilation holes h having a uniform diameter, a fluid such as exhaust gas flows easily. On the other hand, the cordierite shown in FIG. 9B also has pores having various diameters, particularly independent pores (closed pores) hc having a small diameter.

FIG. 10 shows a case in which air at a temperature of 20 (° C.) is supplied at a flow rate Va (m / sec) to an alternate plugged honeycomb filter having a diameter of 33 (mm) and a length of 150 (mm) as a reference dimension. FIG. 5 is a diagram showing an initial pressure loss Pi (mmAq), in which the vertical axis represents the initial pressure loss Pi (mmAq), and the horizontal axis represents the flow velocity Va (m / sec).

In the case of the cordierite alternating plugged honeycomb filter, when particulates are not deposited, large pores constituting the cordierite contribute to a decrease in ΔP3 in the above equation (1), and the initial pressure loss Pi at any flow velocity Va. Becomes smaller. However, when the particulates are slightly accumulated, the pores having a large surface area are covered with the particulates, and the pores do not contribute to the reduction of the pressure loss. In this case, only the closed pores hc and the pores having a relatively small diameter contribute to the reduction of the pressure loss ΔP. Therefore, even if the filter has the same cell structure, the SiC alternating plugged honeycomb filter has a smaller size. However, as shown in FIG. 7, the pressure loss ΔP is reduced as compared with the conventional cordierite alternately plugged honeycomb filter.

Referring to FIG. 7, even with the same SiC alternating plugged honeycomb filter, the cell structure Cs = 17/100 (mil / cpi) pressure loss P1 and the cell structure Cs = 14/200 (mil / cpi). Comparing the pressure loss P2 and the pressure loss P3 of the cell structure Cs = 12/300 (mil / cpi), it can be seen that a filter having a large number of cells and a thin cell wall thickness dc can reduce the fluctuation of the pressure loss.

FIG. 11 is a data diagram showing a pressure loss ΔP caused by changing the diameter φ of the SiC alternating plugged honeycomb filter. However, each of the SiC alternating plugged honeycomb filters has the same cell structure of 14/200 (mil / cpi), and the symbol P4 is the pressure loss and the symbol generated when φ = 144 (mm). P5 is the pressure loss that occurs when φ = 165 (mm), and P6 is the pressure loss that occurs when φ = 190 (mm).

Referring to FIG. 11, it can be seen that even with the SiC alternating plugged honeycomb filter having the same cell structure, as the volume of the filter increases, the pressure loss and its fluctuation can be reduced.

Incidentally, the total volume of the SiC alternating plugged honeycomb filter 22 needs to be set to an appropriate value with respect to the total displacement of the diesel engine. However, if the total volume of the SiC alternating plugged honeycomb filter 22 is too small, the pressure loss ΔP becomes large, and the fuel consumption is significantly deteriorated. Conversely, if the total volume of the SiC alternating plugged honeycomb filter 22 is too large, it may not be possible to install the filter between the exhaust passages 11 due to layout problems.

Therefore, paying attention to the fact that the total volume of the filter is determined by the total displacement of the internal combustion engine, the total volume of the SiC alternating plugged honeycomb filter 22 is set to be 1/4 to 2 times the total displacement of the diesel engine. Is preferred. In this case, according to the present embodiment, the pressure loss ΔP generated between the SiC alternating plugged honeycomb filters 22 can be suppressed, and the exhaust gas purifying device according to the embodiment of the present invention is disposed between the exhaust passages 11. Can solve the problem on layout.

In the present embodiment, a single filter has been described, but a branched structure in which a plurality of filters are arranged in parallel between the exhaust passages 11 may be used. In this case, the filter can be used with the conventional dimensions without increasing the volume of the filter, which is effective for a vehicle with a large displacement.

It is a schematic cross section which illustrates the alternating plugged honeycomb filter for exhaust gas purification. FIG. 4 is a data diagram showing a relationship between a filter cell structure and typical specifications, a geometric surface area, and an aperture ratio of the filter. FIG. 9 is a data diagram showing a relationship between an amount Q of particulates captured per unit volume and a flow velocity Va of air passing through a cross section of the filter. FIG. 3 is a data diagram showing a pressure loss occurring when cordierite is used as an alternating plugged honeycomb filter for purifying exhaust gas and considering the presence or absence of a fuel additive, and a temperature T in the filter as a time t. FIG. 1 is a schematic sectional view showing one embodiment of the present invention. (A) is a perspective view showing an alternating plugged honeycomb filter for exhaust gas purification. (B) is a partial front view showing a part of the cell structure. It is a data diagram which compared the pressure loss which arises in a SiC alternating plugging honeycomb filter at the time of using diesel fuel which an additive contains, and the pressure loss which arises in a cordierite alternating plugging filter. FIG. 4 is a data diagram showing the abundance ratio of pores having different diameters in cordierite and SiC. (A) is a schematic sectional view showing the internal structure of SiC. (B) is a schematic sectional view showing the internal structure of cordierite. FIG. 8 is a data diagram showing an initial pressure loss Pi at a flow velocity Va. FIG. 4 is a data diagram showing a pressure loss caused by changing a diameter φ of a SiC alternating plugged honeycomb filter when a diesel fuel containing an additive is used.

Explanation of reference numerals

11 Exhaust passage 20 Exhaust gas purifier 22 SiC alternating plugged honeycomb filter 23 Exhaust gas circulation hole 23a Small piece 23b Heat insulating material 24 Casing 32 Cordierite alternating plugged honeycomb filter 33 Exhaust gas circulation hole 33a Small piece

Claims (3)

An exhaust gas purifying honeycomb filter that is disposed between an exhaust passage of an internal combustion engine using a fuel additive to reduce particulates in exhaust gas and captures particulates remaining in the exhaust gas,
An exhaust gas purifying honeycomb filter, which is an alternating plugged honeycomb filter formed of a porous silicon carbide sintered body. 2. The honeycomb filter for purifying exhaust gas according to claim 1, wherein the honeycomb filter has a cell structure in which the number of cells per unit square inch is more than 120 and the thickness of a cell wall is less than 0.41 (mm). The honeycomb filter for purifying exhaust gas according to claim 1 or 2, wherein a total volume thereof is 1/4 to 2 times a total displacement of the internal combustion engine.

Images